An Alaskan tyrannosaur is walking under a northern polar night sky, through a mudflat, and encountering a trackway left by a hadrosaur (duck-billed dinosaur). In the distance, an ancient boreal forest outlines the Cretaceous landscape. The Late Cretaceous (100 to 66 million years ago) represents the end of the Mesozoic Era and the height of dinosaur diversity. At the end of the Cretaceous a massive extinction event occurred that claimed all non-avian dinosaurs, pterosaurs, large marine reptiles, several groups of early birds, a few families of fish, and several groups of mollusks such as ammonites. The extinction event also marked the beginning of the Cenozoic Era and the great diversification of mammals.